Writing windows daemons in C++20

One little snippet I’ve found myself reusing surprisingly often is how to write a daemon program with graceful shutdown in windows. To recap, a daemon is a program that sits and does ‘background work’ until it is explicitly shut down by the user. For my purposes, it is also a console program. Like this one:

int main(int argn, char** argv)
{
  while (true)
  {
    std::cout << "ping!" << std::endl;
    std::this_thread::sleep_for(100ms);
  }
  std::cout << "shutdown!" << std::endl;
  return EXIT_SUCCESS;
}

If you run this program, it will, of course, continuously print “ping!”. And you can kill it by entering ctrl+C on the console. But the shutdown will not be graceful: “shutdown!” will not be printed. It’ll just look like this:

ping!
ping!
ping!
^C

C++20 introduced std::stop_source and std::stop_token, which help to implement a graceful shutdown. We’ll use the following code:

'namespace
{
static std::stop_source exit_source;
static std::atomic<bool> main_exited = false;
static bool already_registered = false;

static void atexit_handler()
{
  main_exited = true;
}

BOOL control_handler(DWORD Type)
{
  switch (Type)
  {
  case CTRL_C_EVENT:
  case CTRL_CLOSE_EVENT:
    exit_source.request_stop();

    while (!main_exited)
      Sleep(10);

    return TRUE;
    // Pass other signals to the next handler.
  default:
    return FALSE;
  }
}
} // namespace

std::stop_token register_exit_signal()
{
  if (!already_registered)
  {
    if (!SetConsoleCtrlHandler((PHANDLER_ROUTINE)control_handler, TRUE))
      throw std::runtime_error("Unable to register control handler");

    atexit(&atexit_handler);
    already_registered = true;
  }
  return exit_source.get_token();
}'namespace
{
static std::stop_source exit_source;
static std::atomic<bool> main_exited = false;
static bool already_registered = false;

static void atexit_handler()
{
  main_exited = true;
}

BOOL control_handler(DWORD Type)
{
  switch (Type)
  {
  case CTRL_C_EVENT:
  case CTRL_CLOSE_EVENT:
    exit_source.request_stop();

    while (!main_exited)
      Sleep(10);

    return TRUE;
    // Pass other signals to the next handler.
  default:
    return FALSE;
  }
}
} // namespace

std::stop_token register_exit_signal()
{
  if (!already_registered)
  {
    if (!SetConsoleCtrlHandler((PHANDLER_ROUTINE)control_handler, TRUE))
      throw std::runtime_error("Unable to register control handler");

    atexit(&atexit_handler);
    already_registered = true;
  }
  return exit_source.get_token();
}

You’re going to have to include both <stop_token> and <Window.h> for this. Now we can adapt our daemon loop slightly:

int main(int argn, char** argv)
{
  auto token = register_exit_signal(); // <-- register the exit signal here
  while (!token.stop_requested()) // ... and test the current state here
  {
    std::cout << "ping!" << std::endl;
    std::this_thread::sleep_for(100ms);
  }
  std::cout << "shutdown!" << std::endl;
  return EXIT_SUCCESS;
}

Note that this requires cooperatively handling the shutdown. But now the output correctly prints “shutdown” when killed with ctrl+C.

ping!
ping!
shutdown!

There’s linux/macOS code for this same interface too. It works by handling SIGINT/SIGTERM. But that information is somewhat easier to come by, so I’ll leave it out for brevity. Feel free to comment if you think that’d be interesting as well.